Reactions of carbonyl compounds in basic solutions. Part 25.1 The mechanism of the base-catalysed ring fission of 3,4-diphenylcyclobut-3-ene-1,2-diones

Abstract

The rate coefficients for the base-catalysed ring fission of a series of substituted 3,4-diphenylcyclobut-3-ene-1,2-diones to give the corresponding (Z)-2-oxo-3,4-diphenylbut-3-enoic acids have been determined in 50% (v/v) aqueous dimethyl sulfoxide (DMSO) at 25.0 and 45.0 °C, as well as for a limited series in 70% (v/v) aqueous DMSO. The activation parameters have been calculated. The effect of both mono- and di-substitution of the phenyl groups has been studied and gives a Hammett ρ value of ca. 1.3 in 50% aqueous DMSO at 25.0 °C. The kinetic solvent isotope and solvent effect and enrichment in ¹⁸O-enriched water have been studied. The product composition of the reaction of the monosubstituted compounds gave a ρ value of ca. 0.9. All the evidence indicates a mechanistic pathway which proceeds by a rapid, reversible addition of hydroxide anion to the dione, followed by a benzilic acid-type rearrangement to form a 1-hydroxycyclopropene-1-carboxylic acid. The latter then suffers ring fission to give the final product.